The present invention relates to power regulation, and particularly relates to switch-mode power supplies.
Advances in one area of technology often require commensurate advances in supporting technologies to realize the full benefit of the advance. For example, observers of the microprocessor industry are familiar with xe2x80x9cMoore""s Law,xe2x80x9d which posits that the complexity of semiconductor devices doubles every two years. Microprocessor development arguably represents the most dramatic illustration of Moore""s Law. Pioneering microprocessors released in the 1970""s operated at clock speeds well under 500 KHz, and included fewer than five thousand transistors. Modern microprocessors operate at clock speeds in excess of 1 GHz and include millions of transistors. Exploiting these dramatic gains required advances in a host of supporting technologies, from advances in memory technology and circuit fabrication, to advances in power supply design.
Indeed, modern microprocessors could not provide their dramatic performance gains absent today""s sophisticated power supplies. For example, high-end microprocessors can consume in excess of 80 Watts of power and operate at 2 VDC or less. These requirements translate into power supply output current requirements in excess of 40 Amps, yet the power supply must maintain tight output voltage regulation, even when faced with dramatic step changes in output current. In general, modern electronic systems require responsive power supplies capable of providing relatively clean power at well-controlled voltages, over a wide range of quickly changing load conditions.
Often, the requirements placed on electronic power supplies include the dual requirements of good transient response and high efficiency. Linear voltage regulation, where a pass element such as a transistor, is used to drop a supply voltage down to a regulated value have good transient response, but can be inefficient. Linear regulation inefficiency rises with increasing input/output voltage differential. Because of the high current required by high-performance electronic systems, many primary power supplies provide a relatively high voltage, such as 12 or even 24 VDC. Regulating such voltages down to 2 VDC or less, as is common for high-performance microprocessor cores, is impractical using linear regulation.
Switch mode power regulation offers an opportunity for improved efficiency as compared to linear regulation. In switch mode power supplies, one or more reactive circuit elements are rapidly switched from one configuration to another to control the energy flowing into and out of a reactive circuit element or elements. By using reactive elements for energy storage, switch mode power supplies, sometimes referred to as switching regulators, minimize power losses when converting from one voltage to another. However, switch mode power supplies entail a host of potential problems that sometimes offset their good efficiencies.
For example, the transient response of switch mode power supplies can be compromised depending upon the control topology employed. Generally, the signal being regulated by the switch mode power supply is fed back so that the regulator can adjust some characteristic of its switching operations to maintain the desired output voltage. In certain feedback implementations, relatively slow error amplifiers reside within the control feedback loop, making the switch mode power supply slow to respond to fast load transients. Fast load transients are common in microprocessors and other complex circuits that operate under dynamic conditions.
So-called xe2x80x9cripple-modexe2x80x9d regulators use a relatively simple control feedback loop based essentially on high-speed comparator circuits that compare the regulated output voltage, or some signal proportional to the regulated output voltage to a desired output level. The regulated output signal includes some amount of ripple, arising from the switching actions of the switch mode power supply. Switching control response is improved by eliminating error amplifiers from the control loop, which are relatively slow compared to these comparator-based control loops. A constant on-time controller represents one implementation of ripple-mode control. With constant on-time, controllers, a switching controller generates turn-on pulses of a fixed width at a frequency determined by changing load conditions, and possibly changing input or output voltages.
The present invention provides an apparatus and method for ripple-mode control of a multi-phase power supply circuit. A ripple-mode switching controller provides one or more features, including cross-phase blanking, active current sharing, out-of-bounds compensation, and virtual ripple generation. Cross-phase blanking permits the controller to preserve the desired phase relationship between switching pulses it provides to the multiple output phases, even when it generates overlapping switching pulses at maximum duty cycle operation. Active current sharing compliments ripple-mode control by allowing the controller to balance the amount of load current provided by the individual output phases, based on either trimming the width of switching pulses provided to one or more output phases, or based on trimming hysteretic control points. The controller may also include an out-of-bounds function to suppress switching pulses in over-voltage situations, and virtual ripple generation to enhance ripple-mode noise immunity.
Active current sharing may be adapted to both constant on-time and hysteretic implementations of the controller. With active current sharing, the controller typically designates one output phase as a master phase. The controller then adjusts its phase switching operations to balance the current carried by each remaining output phase with respect to the master phase. This prevents one output phase from carrying an unequal or excessive share of output load current. For constant on-time operation, the controller trims the width of switching pulses provided to each of the slave output phases based on the relative amount of load current carried by each slave output phase. For hysteretic control, the controller increases or decreases the upper and lower hysteretic switching points for one or more of the slave output phases to achieve the desired current balance.
Pulse trimming or hysteretic adjustment concepts may be extended to include compensation for changes in supply or output voltages such that the controller""s switching pulses are compensated for changing conditions. For example, in constant on-time applications, the controller can be configured to adjust switching pulse on-time to maintain essentially the same steady-state switching frequency over a range of supply voltages. Likewise, the controller could compensate switching pulse width to maintain steady state switching frequency for a range of selectable output voltages.
Further extending its flexibility, the controller may incorporate virtual ripple generation. A feedback signal taken from the controller""s regulated output includes both actual ESR-induced output ripple, as well as the actual DC offset value of the regulated output voltage. Virtual ripple generation adds a desired amount of arbitrary ripple to this feedback signal to increase the noise immunity of ripple-mode regulation. Because the arbitrary ripple simply adds to the actual feedback signal, rather than replacing it, the controller preserves its transient response by maintaining its sensitivity to the actual DC and AC components of the regulated output voltage.